Low bit rate voice coding schemes have been proposed wherein blocks of voice samples are coded into a table address and a gain factor. Such coders are known as vector quantizing coders. For further references on the type of coding involved, one may refer to "Fast Code-Excited Linear Prediction (CELP) Coding Based on Algebraic Codes", by J. P. Adoul et al, in International Conference on Acoustic, Speech and Signal Processing (ICASSP) 1987; and to "Code-Excited Linear Prediction (CELP) High-quality Speech at Low Bit Rates", by M. R. Schroeder and B. S. Atal, ICASSP 1985.
Obviously, the CELP coder quality as well as its performances are particularly dependent upon the table contents. Said table contents (i.e. Codebook) is conventionally loaded with statistically set codewords, which does provide a good tradeoff, but is certainly not an optimal solution on a case by case basis.